The automotive glass industry is, so far, highly labour intensive. For example, the preparation of a windshield or side window for an automobile very often requires a hand lay-up of the non-oriented plastic film to the glass. After the plastic film has been placed on the glass and perhaps most of the air bubbles caught between the film and the glass removed by use of a squeegee, a vacuum ring or other device may be placed around the periphery to pull as much air as possible from between the film and the glass. Thereafter, the glass/plastic film combination may be placed into an autoclave which is then pressurized and heated so as to adhere the non-oriented plastic film to the glass under pressure and heat conditions.
Most often, in such a structure the plastic film may be a composite laminar structure of non-oriented polyvinyl butyral together with oriented polyethylene teraphthalate (PVB/PET), or it may be a composite laminar film of non-oriented polyurethane together with polyethylene teraphthalate; or it may be PVB or polyurethane. The polyvinyl butyral (or polyurethane) is a generally soft non-oriented material which is quite often slightly embossed and somewhat opaque because of its surface character. However, the apparent opacity of the PVB disappears when the film surface is smooth, or is made smooth under the action of heat and/or pressure. The polyethylene teraphthalate is an oriented tough film which provides anti-lacerative characteristics to the glass/plastic film structure, because glass will shatter upon impact. The PVB layer adheres the PET layer to the glass, and also provides energy absorption in the event of impact with the composite laminar structure. Thus, the PET layer provides a barrier to protect a person from shattered glass shards in the event of an accident.
It is a characteristic of polymer plastic films as discussed herein that if there are any significantly large air bubbles entrapped in the interface between that film and glass, they may remain after heat and pressure treatment, particularly as in the prior art. However, if very small air bubbles are entrapped, and particularly if the film layer is PVB, they will be absorbed by the PVB. The difficulty is that significant air bubbles at the interface between the PVB and glass, or at the PVB/PET interface, or wrinkles that may occur in the PVB or PET, materially affect the optics of the glass/plastic film laminar structure in that an object viewed through the glass/plastic film laminate structure may appear to be distorted. This may be especially important in the event of use of glass/plastic film laminar structures as side windows of automobiles or other vehicles, since such windows are generally unframed, and the incidence of air bubbles or wrinkles especially near the edges of the glass is not acceptable.
The autoclave procedure to adhere a non-oriented plastic film to glass requires several hours per cycle, and it is not possible until the after the cycle has finished to determine if the glass/plastic film laminate workpiece being produced is acceptable. In other words, there is no determination of scrap (other than breakage) until after the autoclave cycle has been concluded. Moreover, the amount of unusable workpieces which become scrap may be high, so that the prior art autoclave production methods may be significantly inefficient as to the use of labour, the use of time, and particularly as to the use of energy.
In the meantime, the use of glass to which a polymer plastic film has been adhered as a windshield or as a side window of automobiles and the like has been approved, and it is expected that such use may become mandatory. In particular, use of such laminated glass/plastic film structures as side windows is a significant safety step as there is little that can be done to preclude impact of an object against the side window in the event of an accident where there may be relative sideways movement of any object to the side window; whereas the increasing use of seat belts--the use of which is mandatory in most jurisdictions--and the increasing use of air bags may reduce the chance of impact particularly of the head of a driver or passenger against the windshield of an automobile.
Still further, the production of PVB/PET having uniform thickness is difficult if not nearly impossible to achieve, because there may for example be gauge bands produced. Non-uniform thickness of the PVB/PET may again result in distortion which is visible, even if the distortion has a gauge thickness in the order of 0.00001 inch.
The above difficulty--eliminating gauge bands for example--can be overcome by bending two pieces of glass together so that they have essentially the same shape. Thereafter, one of the pieces of glass which is to be used as a side window for an automobile has the film placed on it. Then, the mating piece of glass may be placed on the film as a cover plate, and a vacuum ring or vacuum bag may be used to withdraw as much air as possible. After that, the combination of the glass having the PVB/PET film, and the cover plate of glass resting upon the PET, is autoclaved as discussed above. Finally, the structure is removed from the autoclave and the glass cover plate could possibly be re-used.
The present inventor has noted that the relatively soft PVB (or polyurethane) non-oriented plastic film layer may indeed act as a gauge when a ram or pusher is being used to press the film against the glass. What this does is to permit a slight adjustment of the glass and the pusher ram acting against it, and this phenomenon occurs as a function of the temperature and/or pressure working on the PVB. In any event, because cold or warm PVB has a moderate tensile strength, it is possible to push a ram against the film and to contact it against the glass--the details of which are discussed hereafter--so that when the film and glass are fully contacted in a manner that substantially precludes any large air bubbles to have occurred, the structure may be heated to the laminating temperature of the PVB.
Of interest in the prior art is U.S. Pat. No. 4,772,006 issued Sep. 20, 1988 to Guglielmetti et al. That patent contemplates multi-laminar glass/plastic film structures that may be glass/PVB/glass or glass/polyurethane/glass. However, the patent is particularly directed to an apparatus whose purpose is to grip and transfer a pre-cut and pre-stored sheet of flexible plastic film to the first layer of glass, after which the second layer of glass is placed on top. There is no particular or specific teaching of the steps that then follow: the patent suggests that the stack formation is then brought to an automatic assembly station which is a pressure sealing stage carried out at high temperature and pressure--in other words, an autoclave.
Two Japanese publications, Number 63/71332 and Number 63/71334, each published Mar. 31, 1988 in the name of Ishikawa and Kurita also referred to multiple laminations, in particular where laminations are placed on both sides of glass. However, in all events the processes discussed by Ishikawa and Kurita are carried out in a vacuum chamber or autoclave and result in a laminate structure which is sealed at its edges. Ishikawa and Kurita are particularly concerned with the probability of dust being deposited between the plastic film and the glass.
The present invention may apply to precast glass which is essentially flat, but it is especially adapted for precast glass that is curved such as for use as windshields and side windows of automobiles. Sometimes the contour of such precast glass may be quite complex.
As it happens, in a simple laminated structure having glass together with PVB/PET, the composite layer of non-oriented PVB polymer plastic film together with oriented PET polymer plastic film is applied to the inside surface of the glass--that is, to the concave side of the curve if the precast glass is curved, which will be the inside surface of the windshield or side window when in use.
Thus, the apparatus and methods of the present invention apply to the production of laminated glass and polymer plastic film whether the precast glass is flat, has simple curvature, or a compound curvature.
One of the essential characteristics of the apparatus of the present invention is that at least on of the saddle--or at least the face of the saddle that is next to the glass when the glass is placed in the saddle--the ram, or both, should have good heat transfer characteristics. Thus, when it is desired in practicing the methods of the present invention for the glass to be heated up, heat is transferred to the glass through the saddle quite quickly; and likewise, when it is desired for the glass to be cooled down, heat is extracted from the glass through the saddle quite quickly.
As described hereafter, although it is usual for the heat to be transferred to the glass through the saddle, it is possible to heat and cool the ram, or to heat one of the saddle and the ram and to cool the other. The present invention also contemplates that one of the saddle and the ram--usually the saddle--may be constantly heated, and the other--usually the ram--may be cooled sufficiently as to cause gross heat extraction away from the glass and polymer plastic combination.
Obviously, the surface of the upper side of the saddle must conform to the surface of the outer curvature of the glass so as to fully support the glass. It is possible that the upper face of the saddle may be lined, particularly as discussed hereafter.
In any event, if the surface of the saddle has essentially the same coefficient of heat expansion and contraction as that of glass, then as the glass is heated up and cooled down there is essentially zero slippage of the glass with respect to the face of the saddle. Obviously, as the glass is being heated up and cooled down while the ram is exerting pressure against the upper side of the glass, if the glass has a coeffieient of heat expansion and contraction which is significantly different than that of the saddle, then either the saddle or the glass will expand or contract at a different rate than the other. Since glass has very little or substantially zero tensile strength, although it has quite high compression strength, if the saddle is permitted to expand faster than the glass then there is a possibility that the glass will break as it is being heated because the saddle will have expanded faster. Likewise, if the saddle expands (and contracts) slower than the glass, then there is a likelihood that the glass will break during cool-down. Such breakage may occur because of the friction or stiction between the glass and the surface of the saddle.
It is possible for the surface of the saddle to be coated with a material that permits some slippage, such as silicone or PTFE.
As discussed hereafter, the present invention contemplates that stainless steel--for example stainless steel (compound SS430)--is particularly useful because it has a coefficient of heat expansion which is quite similar to that of glass. Alternatively, an aluminum base plate (or a base plate of other metal having high heat transfer characteristics, such as brass or copper) may be used because of the higher heat conductivity than that of steel, so that the base plate may be heated and cooled using appropriate heaters and cooling fluids and the like, with a stainless steel face being presented against the glass by the top of the saddle.
It is desirable for the surface of the saddle to be machined as closely as possible to the outside curvature or contour of the glass to be placed on it. However, because glass is reasonably flexible, at least ordinary manufacturing tolerances in terms of preparation of the surface of the saddle may be accommodated.
As discussed hereafter, the present invention also contemplates multi-laminar compound structures having two layer of glass with at least one layer of non-oriented polymer plastic film, one layer of oriented polymer plastic film, and perhaps even two layers of non-oriented polymer plastic film. Even in the event of the manufacture of windshields, because of the specific structures of the ram and saddle as discussed in detail hereafter, the apparatus and methods of the invention are more tolerant of mismatched pairs of glass in such multi-laminar structures as contemplated immediately above.